Commercial DVD Playback in Ubuntu 18.04
This also applies to KDE neon installations using the Ubuntu 18.04 base, and possibly Linux Mint 19.x.
sudo apt install libdvdnav4 libdvdread4 gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly libdvd-pkg
sudo dpkg-reconfigure libdvd-pkg
sudo apt install ubuntu-restricted-extras
A lot of online advice suggest doing everything except reconfiguring the libdvd-pkg package, which leaves you without the necessary libdvdcss package installed on the system for most applications to be able to read the commercial DVD. Following all the above steps should install everything needed, and grab the latest libdvdcss package directly from videolan.org.
This is also explained by the VideoLAN devs themselves here.